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Children's pedal generators have a number of applications, from an entertainment in their own right, providing a distraction to keep children off the main generators when running a pedal PA system to educational projects. Whereas alternators are more suitable for generating serious power from an adult pedaller, their inefficiency at lower power means they're not so good for children's generators. Small permanent magnet DC motors such as the ones used for car radiator fans are much more suitable, and although they're not as good for charging batteries the fact that they operate perfectly well without them can be an advantage. For a start a batteryless system is better for education as it's really obvious where the power is coming from if it stops when pedalling does. It's also a great deal safer as only batteries can produce the high levels of current needed to burn out wires.
Radiator fan motors run at the same sort of speed an alternator does; needing 3000 rpm or so to produce 12 Volts. Children's bicycles have much smaller wheels than adult ones so the wheel rim and belt drive used on adult generators won't work as the pulley for the motor would have to be ridiculously small. A friction type drive using something like an old stereo control knob on the motor shaft driven by a wheel with a tyre works quite well. It isn't as efficient as the belt but the increased efficiency of the DC motor makes up for it overall. DC motors are much more efficient than alternators at lower powers as they don't have to use current to produce the magnetic field.

Like the adult generator page, this isn't so much an exact design to be followed as enough information to enable you to make something that works which you can develop and improve yourself.
A good starting point is a worn out small child's bicycle with stabilisers. Stabilisers are ideal to form the basis for a stand if you take the small wheels off. The cheaper type of cycle has plastic wheel bearings which aren't adjustable and often by the time a child has learnt to balance the bearings have worn out. Quite a bit of play on the back wheel is acceptable for a generator. You'll need a tyre; you can always take the one off the front if the back one's worn out or split. This type of cycle usually has a fully enclosed chainguard, and having eliminated the front wheel you could use the old front mudguard as an extra guard for the rear wheel. The stand needs to have a good solid square base, for instance the base of a reception type office chair made of square tubing. Don't make a triangular stand as they can be tipped towards the front. The back wheel needs to be just clear of the ground and the spindle from the front wheel should be mounted at something like the height of the rear one to keep the seat position comfortable.

Next you'll need a small radiator fan motor. They're often thrown away with leaky radiators but are usually OK. Mini Metro ones are a nice size but one from almost any small car could be used. The Metro ones can be opened up by removing two long bolts - it may be worth putting a small bit of engine oil on the bearing bushes and checking that the commutator brushes aren't too worn.
To drive the motor you'll need a knob of 16 or 20mm diameter. If you can't find one in the junkpile, Maplin's still do a range of aluminium knobs. They're meant for a 6mm spindle size but if your motor has an 8mm spindle you can always drill the hole out a bit but remember to take out any grub screws first. Having got the motor with the knob on the spindle, try connecting a 12V light bulb, for instance a 21W indicator one across the motor and holding the knob against the tyre whilst turning the pedals with one hand. You should find that the bulb will light quite brightly without too much effort, proving that you've got the basis for a generator.

If the back wheel is perfectly round with no dents or warps you might be able to get away with a fixed mounting for the motor, but generally it's better to make up a pivoting bracket so the drive knob is held against the tyre by the weight of the motor. If you've got the Metro motor, it's got three lugs which are already in about the right place. All you need to do is make up another one at the front out of a bit of 2-3mm scrap aluminium bolted through the motor with the two long bolts.
A suitable pivot pin can be made out of the non threaded part of a long bolt or just a bit of 8mm rod drilled at the ends for split pins.
Make up a mounting bracket for the pivot out of some scrap metal to make the drive knob lean on the tyre so that a line through the pivot and the spindle would be about 45 degrees from vertical. The motor should lean into the direction of the wheel; that is the drag on the motor should pull it into better contact with the wheel. We've occasionally had problems with motors bouncing under a higher current load; keeping the tyre pressure low or adding an inner tube rubber band to increase the contact pressure usually fixes it. Depending on your layout, it may be necessary to remove unwanted lugs from the motor so children don't catch their heels on them.

If you're going to use the generator in a situation where lots of different children are going to go on it, you will need to modify the seat height adjuster. We usually drill an 8mm hole through the seat post and also a series of holes lining up with it in the saddle stem so the height can be set quickly like an axle stand. The handlebar adjustment isn't usually so bad as it's less critical and there isn't the weight on the handlebars so you can still use the original clamp if you put a handwheel or quick release on it.

When you come to doing the wiring, note that the red wire from the motor may not actually be the plus one, depending which direction the motor's turning. (It won't make any difference to the operation of the motor if you're turning it the 'wrong' way)
Once you've got the motor mounted, have another go at spinning it both with just a voltmeter and with a 21W indicator or stop light bulb as a load. You should find that you can get just over 12V with the bulb on and hit 16 to 18 Volts with no load. If the voltage goes a lot higher than that, you should look at increasing the size of the drive knob to reduce the gear ratio.
The motor behaves quite differently to an alternator; the voltage produced is proportional to the pedalling speed, and you can't damage it by running with no load on.

That's it for the most basic form of the generator; if all you want to do is light up some 12 Volt light bulbs as an energy demonstration or run other things which aren't voltage sensitive, you've got a generator which will do it. For basic lead acid battery charging, connecting a diode such as a 5-Amp rated P600D in series so that the cathode (stripe) end of the diode goes to the battery plus will stop current from going back from the battery to the motor. Once the motor spins fast enough to exceed the battery voltage the drag will increase noticeably and the generator will put two or three Amps in. Obviously you shouldn't put in lots of charge without checking the voltage as there's no regulation. Don't charge small lead acids say less than 20Ah and definitely don't try NiCads as they can explode if overcharged. Charging a 40Ah leisure battery is quite possible provided you check it from time to time with a DVM to make sure it doesn't go over 15V.

While on the subject of safety, although there's virtually no electrical hazard as the voltage can't get high enough for a shock and without a battery there's not enough current to burn wires out, the mechanical hazards are considerable. Everything should be guarded as far as possible and the guards designed so they can't make things worse such as by trapping a finger against a moving part. Remember that a child pedalling can't see another one getting involved in the rear wheel, and might not have the sense to stop pedalling even if they did. Children's generators at a public event should be supervised the whole time they're in use and put away or covered at other times to prevent accidents.

We've developed some displays and things of various technical levels to go with children's generators. A basic one consists of a large voltmeter with a smoothing circuit feeding output terminals, and a row of light bulbs driven by a bit of electronics which turns on more as the voltage increases. The two P600D diodes are there to make absolutely sure the circuit cannot get reverse polarity power from someone turning the wheel backwards. (the freewheel should stop it, but it won't stop kids turning the wheel backwards by hand.)

A popular application for our generators has been the binliner inflatables powered by a 12 Volt fan fixed in a plastic bucket. The idea is very simple; a 12 Volt computer fan rated at 6 Watts and 100 cfm or so will run easily from the generator at voltages from 6 to 18 and inflate a structure made from several bin liners joined with insulating tape. Inflatables are held on to the bucket with rubber bands made by cutting old inner tube into slices. However when we tried fan buckets on the larger chair based generator we found that it was capable of producing enough voltage to damage them when pedalled too energetically. We got around this problem by making a shunt regulator to limit the voltage similar to those used on wind generators and solar panels. The shunt acts as a brake on the generating motor - normally the voltage produced is proportional to the speed of rotation, so when you limit the volts it effectively limits the maximum speed of the motor. The result is greatly increased tyre wear but it's better than burning out the fans, particularly as we discovered that the supply of the original surplus ones we got seems to have dried up! It also makes the generator suitable for charging lead acid batteries.

The fans we used came from an old type of computer UPS which were being broken up by a surplus company (Zia Solar) for the batteries. If you can't find anything similar secondhand, it might even be worth buying one new such as 926-220 from Farnell at a bit under £20 if you've already managed to get eveything else cheap or recycled. It's a good idea to fit finger guards on both sides of the fan - you might have to buy those new as well.

Another popular application is pedal powered computer games. The ideal ones are Gameboys or Master Systems which load a simple game from a cartridge so they're not upset by sudden power interruptions. Older Gameboys run from 6V but the newer ones need only 3V. If you're not into too much home brewed electronics and nervous of blowing up the game console, buying a standard car socket to low voltage adaptor is probably a good way out. However you may have to link it directly into the battery connectors as Nintendo in particular use non standard DC input plugs.